test: cover mutable iterator chains in chunks_exact_to_as_chunks

[cuishuang]

The mutable suggestion for chunks_exact_to_as_chunks was already fixed by #17316, but the exact for_each reproducer reported in #17593 was not covered by the rustfix tests.

This adds regression coverage for:

• mutation through for_each;
• mutation through a map/collect iterator chain;
• the generated .iter_mut() suggestions compiling after rustfix.

Refs #17593

changelog: none

[rustbot]

Thanks for the pull request. A reviewer will take a look after it receives 2 community reviews.

In the meantime, we would highly appreciate if you could try to review any of PRs waiting on community reviews.

[CommanderStorm]

Community review: lgtm

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[DanielEScherzer]

community review: extra tests are good

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[rustbot]

r? @Jarcho

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The reviewer was selected based on:

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